1. Field of the Invention
The present invention relates to a current-voltage converter circuit which processes currents output from photodiodes used as light-receiving devices in, for instance, the optical heads of optical disc apparatus.
2. Description of the Related Art
In the case of the optical head used in an optical disc apparatus, the reflected light from an optical disc is detected by using a plurality of photodiodes. The output currents from these photodiodes are converted into voltages by a current-voltage converter circuit and, by the suitable combination of these voltages, a focussing signal for focussing an objective lens, a tracking signal for tracking the optical disc and an information signal are produced. Here, the bandwidths of the focussing signal and the tracking signal are 0-20 kHz, and the bandwidth of the information signal has a broad bandwidth of 0 - several MHz.
In the conventional current-voltage converter circuit, the outputs currents of first and second current sources, which are composed of photodiodes, are respectively supplied to first and second operational amplifier circuits, and converted into voltage values. The output voltages of the first and second operational amplifier respectively, produce low-pass signals V.sub.1 and V.sub.2 via first and second low-pass filters. The focussing signal and the tracking signal are produced from low-pass signals V.sub.1 and V.sub.2. Also, the output voltages of the first and second operational amplifiers are combined by a synthesizer circuit, and an information signal V.sub.3 is produced.
However, in the above conventional circuit, the output currents of the first and second current sources which include 0 - several MHz signal are directly converted into voltage values by the first and second operational amplifier circuits. Therefore, it is necessary for the first and second operational amplifier circuits to possess broadband characteristics, and the operational amplifier of the synthesizer circuit which combines these output voltages must also have a broadband characteristic. Moreover, since the output currents of the current sources are directly converted into voltage, at least one operational amplifier circuit having broadband characteristics is required for each current source. Generally, the operational amplifiers having broadband characteristics are relatively expensive. Thus there was a problem in that, as the number of current sources increased, the circuit production cost became increasingly expensive.